1. Field of the Invention
The present invention relates to a supercharging pressure control device and, more particularly, to a supercharging pressure control device which is utilized for a supercharging pressure control system of an engine provided with a supercharger.
2. Description of the Related Art
In a conventional engine provided with a mechanical supercharger, a quantity of air for the supercharging is increased in accordance with the increase of the number of revolutions of the engine, so that the pressure within the intake manifold is also increased in accordance with the number of revolutions of the engine. However, when a supercharging pressure is increased too much, durability of the engine is adversely effected, for example, generation of knocking, etc. occurs. Therefore, some methods have been developed for discharging the supercharging air into the atmosphere or by-passing the supercharger when the supercharging pressure exceeds a predetermined value.
FIG. 4 shows a conventional control apparatus of an engine supercharger in Japanese Laid Open Publication No. 58(1983)-101218. Therein, a supercharging control valve 44 is located upstream of a throttle valve 43 in an intake passage 42 of an engine 41. A supercharging passage 45 is formed so as to bypass the supercharging control valve 44. A supercharger 46, driven by the rotation of the engine 41, is positioned in the passage 45. The throttle valve 43 and the supercharger control valve 44 are operatively interlocked or interconnected by a linkage 47. The supercharger control valve 44 is closed in accordance with the opening actuation of the throttle valve 43. In case the supercharging pressure supplied by the supercharger 46 exceeds a predetermined supercharging pressure, the supercharging control valve 44 is opened by an actuator 48 regardless of the position of the throttle valve 43 and the supercharging pressure is maintained at a certain or predetermined value.
The conventional apparatus is operated so as to maintain the supercharging pressure at a certain or predetermined value by the actuator. However, it was not possible to decrease the supercharging pressure in the prior art apparatus in order to decrease fuel expenses.